This invention relates to methods and apparatus for detecting electrical defects in a semiconductor device or test structure having a plurality of features that are specifically designed to produce varying voltage potentials during a voltage contrast inspection. More particularly, it relates to inspection techniques for detecting open and short type defects within the features of the circuit or test structure.
A voltage contrast inspection of a test structure is accomplished with a scanning electron microscope. The voltage contrast technique operates on the basis that potential differences in the various locations of a sample under examination cause differences in secondary electron emission intensities when the sample is the target of an electron beam. The potential state of the scanned area is acquired as a voltage contrast image such that a low potential portion of, for example, a wiring pattern might be displayed as bright (intensity of the secondary electron emission is high) and a high potential portion might be displayed as dark (lower intensity secondary electron emission). Alternatively, the system may be configured such that a low potential portion might be displayed as dark and a high potential portion might be displayed as bright.
A secondary electron detector is used to measure the intensity of the secondary electron emission that originates from the path swept by the scanning electron beam. Images may then be generated from these electron emissions. A defective portion can be identified from the potential state or appearance of the portion under inspection. The portion under inspection is typically designed to produce a particular potential and resulting brightness level in an image during the voltage contrast test. Hence, when the scanned portion""s potential and resulting image appearance differs significantly from the expected result, the scanned portion is classified a defect.
Unfortunately, conventional voltage contrast inspection techniques have associated disadvantages. For example, conventional voltage contrast inspection uses a scanning electron microscope (SEM), which is a relatively expensive tool. Additionally, an SEM inspection has a relatively low throughput and, accordingly, represents a significant bottleneck in the semiconductor manufacturing process.
Accordingly, there is a need for improved apparatus and methods for efficiently detecting defects within a voltage contrast type test structure or circuit pattern.
In general terms, the present invention provides apparatus and methods for optically inspecting a voltage contrast type test structure, product circuit pattern, or the like. An optical image of a voltage contrast type test structure is generated. When the optical image has a first type of intensity pattern, it is determined that there is a defect within the test structure. When the optical image has a second type of intensity pattern, it is determined that there is no defect. The intensity patterns correspond to different voltage potential patterns that are expected to be generated if the test structure were subject to a voltage contrast inspection. That is, the first intensity pattern corresponds to a first voltage potential pattern that would be generated during a voltage contrast inspection when there is a defect present, and the second intensity pattern, corresponds to a second voltage potential pattern that would be generated during a voltage contrast inspection when there is not defect present. For example, if the test structure was expected to have a same voltage potential during a voltage contrast inspection, the test structure may be expected have a corresponding uniform intensity value during an optical inspection. When this example test structure has an intensity pattern which includes different intensity values, it is determined that there is a defect within the test structure.
In one embodiment, a method of optically inspecting a semiconductor voltage contrast type structure that is designed to have an expected pattern of voltage potentials during a voltage contrast inspection is disclosed. An optical image of the structure is obtained. The optical image has a pattern of specific intensity values. When the pattern of intensity values of the optical image fail to substantially correspond to the expected pattern of voltage potentials of the structure, it is determined that the structure has a defect. When the pattern of bright and dark intensity values of the optical image substantially correspond to the expected pattern of voltage potentials of the structure, it is determined that the structure does not have a defect
In a specific implementation, the expected pattern of voltage potentials correspond to a substantially same voltage potential on the test structure. In a further aspect, the pattern of intensity values correspond to the expected pattern of intensity values of the structure when the pattern of intensity values have a substantially same intensity value. In yet another aspect, the expected pattern of voltage potentials correspond to a portion of the structure that is coupled with a substrate. In another specific aspect, the pattern of intensity values correspond to the expected pattern of voltage potentials of the structure when the pattern of intensity values have a same relatively bright intensity value. In yet another implementation, the pattern of intensity values does not correspond to the expected pattern of intensity voltage potentials of the structure when the pattern of intensity values include a bright intensity value and a dark intensity value. In one aspect, the dark intensity value corresponds to the defect, and the defect is a complete or partial defect within the structure. In one implementation, the structure includes a plurality of optically visible upper conductive portions, and the optically visible conductive upper portions are designed to form part of a same via chain which is designed to be coupled to the substrate.
In an alternative implementation, the expected pattern of voltage potentials include a first voltage potential and a second voltage potential that significantly differs from the first voltage potential, the first voltage potential being adjacent to the second voltage potential. In a further aspect, the first voltage potential corresponds to a first substructure of the structure that is coupled with a substrate and the second voltage potential corresponds with a second substructure of the structure that is not coupled with the substrate. The first substructure is adjacent to the second substructure. In another aspect, the pattern of intensity values correspond to the expected pattern of intensity values of the structure when the first substructure has the first intensity value and the second substructure has the second intensity value. In another embodiment, the pattern of intensity values does not correspond to the expected pattern of intensity values of the structure when the first substructure has a same intensity value as the second substructure. In yet a further aspect, the same intensity value corresponds to a short between the first and second substructures. In one aspect, the first and second substructure each includes a plurality of optically visible upper conductive portions, and the optically visible conductive portions of the first substructure is designed to form part of a same via chain which is designed to be coupled the substrate.
In another embodiment, the invention pertains to an optical inspection system for optically inspecting a semiconductor voltage contrast type structure that is designed to have an expected pattern of voltage potentials during a voltage contrast inspection. The system includes a beam generator for directing an incident optical beam towards a sample surface and a detector positioned to detect a detected optical beam originating from the sample surface in response to the incident optical beam. The system further includes a processor arranged to perform one or more of the above described method operations.
These and other features of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.